Archive | August 2022

The Rest of the Story: The Harz Mountains

This post is a continuation of previous posts on northeast and central Germany, but we won’t be seeing direct evidence for glaciers in the Harz Mountains (Fig. 1).

Figure 1. Aerial view of the Harz Mountains (from Wikipedia).

Today’s post discusses some of the rocks exposed in the valleys, gorges, and road cuts that dissect the Harz Mountains (Fig. 2).

Figure 2. The large circle indicates the Harz Mountains (inset map). Locations, A through D, are approximate sites of photos and rocks discussed in this post.

We approached the Harz uplands from the east (site A in Fig. 2), where we encountered mines based on removing sedimentary rocks for use as building material (Fig. 3).

Figure 3. (A) Road and building gravel mine from the eastern end of Harz Mountains (site A in Fig. 2). (B) Close-up of tailings pile, showing uniformity of the conglomerate being removed from the open pit.

The mines in Fig. 3 were removing desired beds from the Tanner Graywacke ( age ~360-320 Ma), a poorly mixed sedimentary rock (e.g. graywacke) originally deposited in ocean trenches associated with volcanic island arcs. The Tanner graywacke ranges from mudstone to conglomerate. It forms medium beds with variable texture, and has been tilted to varying degrees (Fig. 4).

Figure 4. Photos of Tanner Graywacke near site A (see Fig. 2 inset for location). The beds in (A) are tilted about 30 degrees (unknown direction) and the outlined layer is ~6 inches thick. Panel (B) shows two nearly horizontal beds exposed within a kilometer of (A). These beds contain detailed sedimentary structures, like cross-beds, as indicated by white outlines in the lower bed.

Our route east of the Harz Mountains (red line in the inset of Fig. 2) took us to site B, where we encountered more facies of the Tanner Graywacke (Fig. 5).

Figure 5. Tanner Graywacke exposures at Site B (see inset of Fig. 2 for location). The massive layers in (A) are ~4 feet high. A thick layer of conglomerate (panel B) doesn’t form cliffs. Note the irregular cobbles exposed by weathering. (Image B is about 6 feet high.)

Our path (red line in Fig. 2 inset) led us into a valley between the small highlands where Figs. 4 and 5 were taken. This valley is filled with a lake, and probably follows a fault zone between Site B and the main Harz Mountains to the north (Fig. 6).

Figure 6. View looking north towards the Harz Mountains. This area is underlain by marine evaporites, including salt that occurs in domes (age ~250 Ma). Extensive karst development in carbonates has led to serious subsidence problems in the area as sinkholes continue to develop.

Our journey followed the southern margin of the Harz Mountains (red line in Fig. 2 inset), taking us by Site C, where we found nearly horizontal beds of Tanner Graywacke exposed along road cuts. We couldn’t stop until we found a rest area, where a large block was available for close examination (Fig. 7).

Figure 7. Images of a block of Tanner Graywacke (A) exposed at site C (see Fig. 2 for location). The boulder was not in place nor was it a glacial erratic. It was put their during highway construction. (B) Close-up of the weathered surface, showing a bright-white square in the circle; this is a grain of Na-feldspar encased in a matrix of clay minerals (weathered to black in the photo). Note the large, pinkish form in the extreme upper-right of the image. This is a block of what was probably a granitic source rock. (C) Lower magnification image of the same rock; the circled area includes dark spherules against a white matrix, which I cannot identify. If you zoom in closer by opening the image, you will see that they are actually rectangular and have smooth edges. The white weathering product is probably from feldspars whereas the dark minerals (entire crystals) may be amphibole or pyroxenes (more resistant to weathering). It is important to recall that a graywacke collects near the source, and thus includes minerals in every size and shape, and every stage of weathering. (D) The circle highlights a cavity that was occupied by a large piece of rock (instead of a mineral grain), similar in shape to the phenocryst displayed in plate (B).

We continued around the western end of the Harz Mountains and found exposures of the marine deposits (including evaporites and carbonates) that underlay the town of Kelbra (Fig. 6), including a thick sequence of either salt or anhydrite (Fig. 8).

Figure 8. Images of the marine sequence (age ~250 Ma) that is much younger than the Tanner Graywacke (age ~360-320 Ma), taken at approximately Site D (see Fig. 2 inset for location). (A) Contact between an evaporite (white rock) and overlying sediments (tan), showing several anomalous features. For example, there is some suggestion that the darker beds have been folded (zoom in on the image); several irregular blebs of evaporite (e.g. to the right of image) appear to be isolated. This may be a salt diapir (or some other ductile rock) that forced its way into younger sediments, folding them as it intruded. A fault zone is not out of the question, considering that site D is located at the margin of the Harz upland (compare to Fig. 4A). Plates B and C show medium beds (<1 foot in thickness) of resistant siltstone surrounded by mudstone/calcarenite. These beds may be tilted but not at such extreme angles as suggested by plate A.

This post reveals rocks that are widely separated in time while being found near each other, supporting the Harz uplift as they do (Fig. 2). As the title of this post suggests, geology is not a series of isolated events. Let’s get the rest of the story.

The Tanner Graywacke was deposited in an island arc, a tectonic region in which oceanic crust is being subducted beneath either a continental (or less often an oceanic) tectonic plate, about 350 million years ago. What was happening on the opposite shore of this proto-Atlantic ocean (aka Iapetus)?

I have encountered rocks of similar age in northern Virginia and discussed them in previous posts. On the west side of Iapetus, during a mountain-building event called (in America) the Acadian Orogeny, a series of island arcs were being subducted/accreted to form a series of suspect terranes. This orogeny was only a phase of the collision of Laurentia (porto-north America) and Avalon (proto-Europe), which endured for most of the Paleozoic era.

The next problem is what happened during the ensuing 100 million years, between deposition of the Tanner Graywacke and the evaporites and carbonates we encountered west of the Harz uplands (Fig. 8)? The collision was completed and Pangaea had been born of two continents…

The mountains rose and they were eroded almost as quickly by wind, rain, and ice, creating massive layers of sediment to the east (modern Europe) and the west (e.g. the Catskill Delta in New York). By 230 million-years ago, the earth’s upper mantle changed its mind and tore the newly formed supercontinent apart, creating rift valleys like that of East Africa in what is now Virginia. Splitting a continent can take as long as building one, but this was a relatively rapid event in geological time; by 200 million-years ago, diabase dikes were injected into the sedimentary and metamorphic rocks created by the closing of the porto-Atlantic Ocean (Iapetus) and the split was well under way. Alluvial and fluvial sediments were collecting in isolated basins in what is now Virginia, and evaporites were settling to the bottom of lakes and brackish coastal waters in Europe, as the ocean invaded…

Jump ahead 200 million years…

Figure 9. View looking upstream in the Elbe estuary, less than a hundred miles from the German port of Hamburg.

I love it when I can understand what the rocks are telling me…

The Forest and the Trees: Glacial Topography of the Central German Plain

Figure 1. Typical scene driving between Usedom and Goseck (See Fig. 2 for approximate location). The hills are glacial moraines as we saw at Usedom in the last post.

The wind blows pretty steady over the North German Plain and Central Uplands, so there are wind turbines everywhere, scattered among the hay fields (Fig. 1).

Figure 2. Map of central Germany, showing stops discussed in this and the next post. The red line is the (very) approximate path taken for this post.

The topography of the Central Germany Plain is very similar to Nebraska, Kansas, and the Dakotas, because they were all created by the advance and retreat of multiple glaciers during the Pleistocene Ice Age. As we saw in the last post, advancing ice sheets as thick as a mile push rock and soil in front of them, before melting back for a few millennia, leaving piles of soil and boulders behind. They are like gigantic bull dozers.

Figure 3. Gentle slope north of Berlin. This is a ground moraine, pulverized rock forming a thin layer of rich soil just waiting for the plows of modern farmers.
Figure 4. The hills in the background are either terminal moraines or eskers (another German word). See text for explanation.

As ice slides over the landscape, scraping off whatever gets in its way, the ice at its base can melt from the friction, creating streams that transport already ground-up rocks. The usual rules of sedimentology apply to these ice-encased streams. They can deposit their sediment load as eskers, which identify these sub-glacier streams, or as piles of scraped-off soil (terminal moraines). I don’t know which I’m looking at in Fig. 4, but this image gives a good impression of their impact on the landscape. Keep in mind that the ice was approximately ONE-MILE thick above the landscape shown in Fig. 4…

Figure 5. Entering the Saale River valley near Goseck.

As I alluded to in the previous post, geology isn’t a sequence of static processes; there is always more than one cause of what we see today, and none of them are stationary. Thus, the landscape produced by the continental glaciers that advanced over the Central German Plain during the Pleistocene were constantly in competition with alpine glaciers created in the valleys and peaks of the Alps. The huge ice sheets had the power to overcome any obstacle…but they couldn’t surmount the steep slopes of the Alps.

The glaciers that originated in the Alps waited until the last retreat of the great ice sheets that originated in Scandinavia, before they could make their play in the Holocene. Vast quantities of easily weathered feldspar were washed down their steep slopes into a panoply of rivers, which cut through the moraines left behind during the retreat of the continental ice sheets, creating broad river valleys like that of the Saale River (Fig. 5). Germany’s central plain and uplands were cut to ribbons by these growing streams, resulting in one of the most water-navigable regions in the world.

You always have to watch your back…

An Erratic Path: Glacial Geology in Usedom, Germany

This post finds me on the Baltic Sea, although I never actually saw it first hand. But this report isn’t about coastal geology; instead, I will be talking about an unusual feature of glacial terrains.

Figure 1. View of the lagoon that separates the island of Usedom from the mainland. An elongate lagoon is surrounded by marsh grass and is connected to the Baltic by narrow channels.
Figure 2. Map of northern Germany. Usedom is the island indicated by the pin. Figure 1 was taken on the west side, where the island is accessed via a short drawbridge.

I haven’t explicitly described glacial terrains in previous posts, and this is not going to be a summary. As always, I’m only going to discuss what I saw with my own eyes. The flat, poorly drained topography of glacial areas (Fig. 1) is often interrupted by linear mounds of loose gravel, sand and silt. These features are called moraines and they are ever-present in northern Germany, especially in Usedom (Fig. 3).

Figure 3. A glacial moraine is seen in this image, in the background, expressed as a low hill, but it is entirely composed of unconsolidated soil.

The primary glacial feature I encountered on this trip is the titular Erratic–large, rounded boulders scattered around a featureless landscape (Fig. 4).

Figure 4. Several glacial erratics have been collected and used as curb markers in this rest stop in Usedom. These boulders have been transported hundreds of miles and have no local source, thus the name erratic.

Let’s look at a couple of examples and see what they tell us about their source.

Figure 5. Photos of a boulder from Fig. 4. The left image shows the appearance of the stone in the field, and the right photo is a close-up (~4X magnification). See text for discussion.

The first erratic boulder I found (Fig. 5) contains no more than 5% quartz (left photo caption), and is dominated by K-feldspar, which is unusual. The magma from which this rock formed (deep beneath the surface where it cooled for millions of years) didn’t contain very much water, which is indicated by the small amount of quartz. The chemistry of the magma is quasi-frozen in the minerals, the second-most-abundant of which is Na Feldspar. The feldspars form a continuum that depends on the relative abundance of potassium (K), sodium (Na), and calcium (Ca); K and Na both form lighter-colored minerals whereas Ca forms dark feldspar minerals. Based on the mineralogical composition of this rock (inset in left image of Fig. 5), this would be classified as a syenite (middle left side of Fig. 6).

Figure 6. Classification of granites based on Na-K feldspar (A point), Ca feldspar (P corner), and quartz (labelled Q). We don’t need to worry about the bottom half of the plot because those are very rare minerals.

Syenites are formed in thick, continental crust. An example today would be the Alps (far beneath them) or the Himalayas, where subduction of denser oceanic crust is not occurring. In other words, the rock shown in Fig. 5 was created deep beneath the surface (~30 miles) when continents collided.

Figure 7. Photos of another erratic from the same location as Fig. 4. Note that this rock (left photo) has a less-reddish hue, an impression supported by the mineralogy (right image). K-feldspar comprises ~15% of the minerals, as opposed to 60% in Fig. 5. The increased quartz content (inset in right image) would classify this as a quartz-syenite; more water was contained in the original magma, but not enough to form significant quantities of quartz.

The boulders seen in Figs. 5 and 7 could have come from the same magma chamber because, as you would expect, there would be variations in local chemistry in a magma chamber tens of miles in diameter, and slow rates of convection wouldn’t mix the magma to a uniform consistency, even over millions of years. Magma, even when heated to 2000 F and buried tens of miles beneath the surface, is still thicker than molasses; it doesn’t mix well.

Figure 8. Photos of another erratic found along the road (Fig. 4), revealing a very different texture from Figs. 5 and 7. This sample contains large (~1/2 inch) irregular clumps of what looks like K-feldspar (note the reddish hue). The overall mineralogical composition (inset of left image) suggests that this rock came from the same magma chamber as the quartz-syenite in Fig. 7.

Phenocrysts like those seen in Fig. 8 are created in intrusive igneous rocks when they go through a multistep cooling process; for example, magma near the edge of the magma chamber loses heat to the surrounding rock and forms crystals like those seen in Fig. 8. These phenocrysts are then captured by the still-molten components of the magma and dragged along for probably hundreds-of-thousands of years (at a very slow speed, like inches per thousand years).

When the magma finally cools enough to become solid rock, it is uplifted as overlying rocks (of all kinds) are eroded by wind and water, not to mention ice. They are finally exposed in great mountain ranges like the Himalayas, where the rock breaks into smaller-and-smaller pieces along joints. When these pieces become small enough to be transported at the base of glaciers (you’ve heard the phrase glacially slow), they are dragged along, scraping over more rocks, sand, and gravel, which leaves evidence of their precarious journey (Fig. 9).

Figure 9. Photos of a glacial erratic from Usedom (not Fig. 4), showing striations that indicate it was dragged across a (rock) hard substrate.
Figure 10. A glacial erratic similar to that shown in Fig.9, but with striations (formed by movement at the base of glaciers) that flow into a joint (circled), suggesting that it was sand and not bedrock, over which this boulder traveled during the most arduous part of its journey. (Sand is mostly quartz, which is much harder than steel or window glass.)

This post has been erratic, starting out looking at a glacial terrain (Figs. 1-4), then taking a detour into igneous petrology, the chemistry of magmas, and mineralogy, with a little plate tectonics thrown in. That’s how geology is; everything is an ongoing process that never quite reaches equilibrium (e.g. the phenocrysts in Fig. 8), and the journey is unending.

I didn’t investigate the origin of the syenite boulders examined in this post, but (if memory serves) they match the mineralogy of intrusive rocks from Sweden, which is a long way from Usedom.

Stockholm is about 500 miles north of Usedom…

Eidersperrwerk: Keeping Out the North Sea

My last post explored the mud flats bordering the North Sea in northern Germany, where we found conflicting methods applied to control and protect the levee system. This post investigates more aggressive measures implemented at the mouth of the Eider River. We will briefly look at the Eidersperrwerk, a gate system designed to control both storm surge incursion up the Eider, and river outflow

Figure 1. Aerial view of the Eidersperrwerk gate system, looking southward. The North Sea is to the right. We will examine the mud flats to the right of the roadway in this post. Note that more than half of the original mouth of the Eider River has been blocked by the levee.(Image from Wikipedia.)

We will focus on the seaward mud flats in this post. Let’s take a look at the south side of the river first (upper-right of Fig. 1).

Figure 2. Sediment retention fences on south side of Eider River. Note the erosion at the base of the fence running across the image from left to right (perpendicular to shore). They are intended to trap sediment, but that doesn’t appear to be happening.
Figure 3. Detail of gate on river side. The gates were closed when we visited at low tide, possibly to keep water depths navigable in the estuary.
Figure 4. Lock approach from seaward. This small enclosure was constructed with steel plates, but the lock gates were closed except to allow the passage of a tour boat, during our study.
Figure 5. Looking north along the seaward side of levee (lower part of Fig. 1). Note the grass that has filled in where only mud was present before (presumably, Fig. 1 is an older image).
Figure 6. Close-up near the junction of levee and lock enclosure (see Fig. 1 for location), showing clumps of grass (center of image) surrounded by pieces of stone used to armor the levee. This looks like recent erosion to me, because the grass grass was probably contiguous with the thick growth near the levee toe.
Figure 7. View looking north in the natural embayment (see Fig. 1 for location). Note the drier sediment near the top of the image. This is a berm that is semipermanent, formed of silt and minor sand by tidal and wave action. It is cut by multiple rivulets, formed as the tidewater drains from the nearshore area (to the right). The scattered boulders are evidence of intense erosion during storms.
Figure 8. View seaward from tip of lock embayment, showing eroded riprap, vegetation clinging to the toe of levee, runnel at low tide (strip of water running north-south in Fig. 1), and berm from Fig. 7 turning seaward.

Comparing Fig. 1 to Figs. 2 and 5-8, we can see the effects of years (probably decades), during which interval the northern margin of the river mouth filled with sediment and grass was established (Fig. 5). Subsequently, it seems that erosion removed some of this soil and grass (Fig. 6). Meanwhile, storms have been slowly wearing away the boulders armoring the base of the levee (Fig. 8) and a semipermanent fair-weather berm was constructed (compare Figs. 1 and 7).

In summary, something appears to have changed in the dynamic environment around the mouth of the Eider. It should come as no surprise that constructing a gate system and cutting off a major sediment supply for at least half the time had dramatic effects on the nearshore. Mud flats are very sensitive to sediment supply, and it could have been either reduced alongshore transport from the north, or the almost-complete denial of rive-borne mud that led to the current situation.

Some scientists propose that storminess varies on many scales, from decadal to millennial as climate fluctuates…

Coastal Restoration on the North Sea

Figure 1. Sign introducing the coastal area and the restoration project. The mud flat here is a mile wide (estimated) because of about 20 feet of tidal range, twice a day.

Today’s post takes me to the North Sea coast of Germany, the city of Husum, and to one of the famous mud flats from the region. Rivers running from the Alps drain Germany, transporting mud (silt and clay) to the north coast, where it is transported along the coast and stirred around by strong tidal flows. We are going to look at efforts to stop dramatic erosion caused by a reduction of sediment input, because of dams and coastal construction, leading to a serious threat to the levee protecting Husum from the North Sea (Fig. 2).

Figure 2. Photo of levee that protects the city of Husum from the North Sea. (Right side of inset map of Fig. 1) The building to the right is an abandoned hotel inside the levee. The building to the left is a restaurant on pilings where people swim during high tide. The asphalt road is the path to the seashore.

The mud flats schematically shown in Fig. 1 are covered with fence-like structures designed to catch mud brought in the the high tide (Fig. 3).

Figure 3. Image of nearshore area (covered by grass), sediment retention fences, and reinforcing riprap where erosion occurs. Note that in this image, the fences do not appear to be collecting sediment on the landward side (to the right).

A quick look at the past. This area was covered by glaciers that filled the North Sea and transported rocks from Sweden to the north. These glacial erratics are rounded and scattered around the land in a random manner (thus the name). We found one used as street decoration in Husum (Fig. 4).

Figure 4. Close up of glacial erratic left along the coast. The boulder was about 3 feet in diameter. This close-up shows muscovite (shiny minerals), orthoclase feldspar (pink), amphibole and/or biotite (dark), and quartz (gray). This granitic rock was transported as much as hundreds of miles by ice, from Scandinavia.

In addition to boulders transported during the ice ages (less than a million years old), there are remnants of sandy sediment from the Quaternary, before the area was overwhelmed by mud (Fig. 5).

Figure 5. Image to the north in Fig.1, showing trees and a village on top of a low pile of quaternary sand, probably the erosional debris of a stream or coastal beach from the last ten-thousand years. This photo was taken to the east side of Fig. 1. Note the sporadic filling by grass, especially the sheep. This is interesting because sheep eat grass, so why they are loose in an area supposedly being reclaimed is confusing.

The result of the sediment retention project can be seen in Fig. 6.

Figure 6. The landward limit of the fencing project, less than 100 yards from the levee. Note erosion along the fence, leaving it standing 2 feet above the exposed mud. This could have been the result of long-term erosion, or a single storm.

This are represents an attempt to reconcile the problem of coastal development (the port of Husum ships out grain) and the protection from storm waves provided by a wide mud flat (which dissipates wave energy). Another issue is the encroachment of sheep grazing, which appears to be legal (there are fences and gates, etc). And then there is entertainment; this is a popular swimming location during high tide. Not to mention environmental degradation and fish hatcheries. Several attempts at mixing these applications can be seen in the hardened and dredged channel leading to the port (Fig.7), and buried groins which were apparently intended to keep the shipping channel open (Fig. 8).

Figure 7. Shipping channel to the port of Husum hardened by mortared rock.
Figure 8. Groin in mud flat. These coastal engineering structures are designed to prevent sediment being carried along the coast and blocking channels, as well as retaining sediment between adjacent groins. This is probably contributing to the erosion seen in Fig. 6.

It is difficult to reconcile the many uses the coastline is required to fulfill. This trip revealed that it is unreasonable to mix methods designed to preserve the status quo (Figs. 7 and 8), and those intended to change it (eg. Fig. 6), especially when these techniques are mixed (Fig. 3). A difficult decision will have to be made soon, or the levee protecting the bustling cit of Husum will be in danger of breach during a severe storm, which is becoming more common in the North Sea.

The Last Few Miles

Figure 1. View looking uphill, along a small ravine in Rock Creek Park. This is a small tributary that shows what this post is about: the stream bed is interrupted by layers of rock every few yards.

This is going to be a brief post, mostly because it is very difficult to convey what I want to communicate in photographs; the camera lens (on my iPhone) simply doesn’t capture image depth well. For example, Fig. 1 was actually pretty steep, but it looks as unintimidating as my driveway.

Figure 2. Topographic map of Rock Creek Park. Note the steep gullies leading to Rock Creek from the west (indicated by dark shading). Figure 1 was taken in the deeply incised terrain east of the Nature Center (top-left of image).

I’ve been talking about the bedrock exposed along the bed of the Potomac in several posts (e.g., Geological Bottleneck and Great Falls), but those are specific locations. Those significant drops in river elevation are part of a larger pattern, one that is displayed even at the scale of Fig. 1. It doesn’t take much of a drop to generate enough potential energy to spin a waterwheel (Fig. 3), which can do a wide variety of work–from grinding corn, to operating a machine shop.

Figure 3. Waterwheel at Peirce Mill used to grind grains like wheat and corn into meal, constructed in the early 1800s, at the lower part of Rock Creek Park (bottom-center of Fig. 2). The stream’s flow was subdivided by channels like that seen in the left of the image to supply water to several mills in the area.

The staircase structure of streams along the transition from crystalline rocks to coastal plains (aka the Fall Line) is so important to the ecosystem that artificial barriers were constructed within the park to ameliorate the impacts of road and bridge construction (Fig. 4).

Figure 4. View looking downstream from a bridge near the top of the map in Fig. 2, showing blocks arranged to replicate the natural steps as seen in Fig. 1. This construction was completed to reintroduce the herring migration. They spawn in the upper reaches of Rock Creek.

Rock Creek National Park deserves its name, not just because of its rock bed. Cambrian sedimentary rocks exposure along the steep tributaries leading to the creek (river?) suggest that bedrock lies not very far beneath our feet (Fig. 5).

Figure 5. Large exposure of Cambrian sedimentary rock formation (image height is about 20 feet), consisting of interlayered sand, siltstone, and shale. Where sand is the predominant component, blocky outcrops like this occur. Siltstone and shale produce more fissile outcrops.

Water has been struggling with rocks for the last 200 million years, always trying to reach the sea. It exploits every nook and cranny in the bedrock until it forms a stream, then a river, and it cannot be stopped. Thanks to the perseverance of water, driven by the steady pull of gravity, the first European immigrants to North America were able to establish a toe hold on what was (to them) a new land…

Ball’s Bluff Battlefield Requiem

Figure 1. A Union artillery piece facing the Balls Bluff battlefield in its approximate position during the battle.

I reported on the geology of this area in a previous post, but I didn’t have much time to explore the area on that outing, so this trip I followed trails all the way around the park. This was the site of a battle early in the American Civil War, October, 1861. The cannon (Fig. 1) is a metaphor of how geology is always in front of us; it isn’t just about really old rocks, but also rivers and beaches, even gas and lava being belched out by volcanoes. That’s all geology too. For example, this battle took place in a field (Fig. 2) .

Figure 2. View from the Union artillery position. It wouldn’t have looked that different in 1861; instead of mowed grass, the field would have been filled with stubble from the recent harvest.

There isn’t much arable land along the Potomac River here because of the rocky soil, but there are a few pockets of land suitable for farming–flood plains left as reminders of the ancient river’s meandering, while it cut its way through rock, gravel, and mud to reach its current position (Fig. 3).

Figure 3. Google Map image of the study area. Our path took us from the end of Ball’s Bluff Road to the southern edge of the map along an inland route. We then followed the bluff (indicated by dark shading) to the north, following the river to the ravine that leads to the Veterans Park trailhead. We cut back to the south, following the gully NNW of the battlefield marker.

The bottoms of the gullies were paved with tilted layers of sandstone and siltstone (Fig. 4), sediment originally deposited in intermontane basins like those that occur in western North America (Fig. 5).

Figure 4. Photo at the bottom of the southernmost valley seen in Fig. 3. Layers of sandstone and siltstone form ledges like this, spaced very hundred yards or so, along the creeks that feed the Potomac river.
Figure 5. Image from the summit of Piestewa Peak in Phoenix, Arizona. The Ball’s Bluff Formation was originally deposited in a similar setting. Sand, silt and clay would have been washed down from local peaks that were probably composed of rocks like the schists comprising the Phoenix Mountains. (Think the Precambrian schists that outcrop along the Potomac River.)

I’d like to finish this post with a thought experiment: Imagine the sediments being carried away from the camera in Fig. 5, passing into the distance to collect in the wide valley that fronts the major fault-block mountain range, seen in the distance; now, imagine everything you see in Fig. 5 being worn down by water and wind and ice, until the sand and silt filling the lowlands in front of the camera is buried beneath the erosional product of Piestewa Peak; imagine that pile of sand and silt and clay being buried many miles beneath the surface, for millions of years.

Can you imagine the rocks seen in Fig. 4?